1. Field of the Invention
The invention relates generally to sonic well logging. More particularly, the invention relates to methods and systems for sonic log data processing.
2. Background Art
The oil and gas industry uses various tools to probe formations penetrated by a borehole in order to locate hydrocarbon reservoirs and to determine the types and quantities of the hydrocarbons. Among these tools, sonic tools have been found to provide valuable information regarding formation properties. In sonic logging, a tool is typically lowered into a borehole, either after the well is drilled or while the well is being drilled (LWD/MWD), and sonic energy is transmitted from a source into the borehole and the formation. The sonic waves that travel in the formation are then detected with an array of receivers. A typical sonic log can be recorded on a linear scale of slowness versus depth in the borehole, and is typically accompanied by an integrated-travel-time log in which each division indicates an increase of one microsecond of the total travel time period. Sonic logs are typically used as direct indications of subsurface properties or—in combination with other logs or other knowledge of the subsurface properties—to find subsurface porosity and other parameters which cannot be measured directly.
Various analysis methods are available for deriving formation properties from the sonic log data. Among these, the slowness-time-coherence (STC) method is commonly used to process the monopole sonic signals for coherent arrivals, including the formation compressional, shear, and borehole Stoneley waves. See U.S. Pat. No. 4,594,691 issued to Kimball et al. and Kimball et al., Geophysics, Vol. 49 (1984), pp. 264-28. This method systematically computes the coherence (C) of the signals in time windows which start at a given time (T) and have a given window moveout slowness (S) across the array. The 2D plane C(S,T) is called slowness-time plane (STP). All the coherent arrivals in the waveform will show up in the STP as prominent coherent peaks. The three attributes of the coherent peak are the peak coherent value (COPK) and the peak location in the slowness-time plane (DTPK and TTPK). The attributes of these prominent coherent peaks are the condensed information extracted from the recorded waveforms. The attributes show the coherence, arrival time, and propagation slowness of all prominent wave components detected from the waveforms.
While the STC method has been very useful in deriving formation properties from sonic data, this method is most useful with non-dispersive waveforms (e.g., compressional and shear waves). This method is not optimal for dispersive waveforms (e.g., Stoneley waves and flexural waves). For processing the dispersive waveforms, the dispersive slowness-time-coherence (DSTC) method disclosed in U.S. Pat. No. 5,278,805 (assigned to the present assignee and incorporated by reference in its entirety) is preferred.
The DSTC method can process the quadrupole signals for formation shear slowness from LWD sonic tools. This is a model-based method in which a set of model dispersion curves are used in the processing to determine which model dispersion curve maximizes the semblance of the back-propagated signals. The formation shear slowness is one of the model parameters that are used to generate the set of dispersion curves. The formation shear slowness parameter value corresponding to the highest coherence peak from the best match dispersion curve would be output by DSTC as the shear slowness.
The DSTC method may use a concentric cylindrical layer model to represent an LWD sonic tool centered in a fluid filled borehole within a uniform formation. The use of a simple concentric cylindrical layer model is for convenience, but it is not essential. If necessary, a more complex model may also be used. The model dispersion curves used in the DSTC method depend not only on formation shear slowness (DTs), but also on other model parameters: formation compressional slowness (DTc), formation density (ρb), mud slowness (DTm), mud density (ρm), hole diameter (HD), the equivalent outer diameter of the tool (OD)—assuming the tool ID is fixed, collar density (ρst), collar compressional slowness (DTc_st), and collar shear slowness (DTs_st). The current DSTC method assumes all these parameters are known and uses them to generate a set of dispersion curves as function of DTs. The first few parameters are related to formation and borehole properties (i.e., formation-borehole parameters) and the last few parameters are related to the collar (collar parameters). For a given size collar, the collar parameters are constants, which can be measured or pre-calibrated. On the other hand, the formation-borehole parameters are variables, changing from depth to depth and from well to well. Therefore, the formation-borehole parameters often need to be determined in order to have an accurate model for the DSTC method.
The formation-borehole parameters can be determined (estimated) from other LWD or offset well wireline measurements. Therefore, these parameters are often available at the surface (uphole). However, not all the formation-borehole parameters are available at all times. The LWD or offset well measurements may be performed after the sonic waveforms are acquired. In this case, the formation-borehole parameters will not be available for real-time processing of the sonic waveform. Even when the LWD data is measured at the same time as the sonic waveform measurements, there may be a delay before the LWD real-time data are available, especially if these other LWD tools are disposed above the sonic tool in the bottom hole assembly (BHA). The unavailability of the formation-borehole parameters downhole would prevent real-time DSTC processing of the sonic waveform data because it is impractical to send sonic waveforms through telemetry uphole during LWD time.
The DSTC method relies on the dispersion curves that correspond to the formation-borehole parameters. The dispersion curves needed for DSTC processing are usually obtained via interpolation from a large database of pre-computed dispersion curves. In the downhole environment, limitations on the downhole processor typically preclude the storage of large databases and any substantial computational load. This makes it difficult to perform real-time DSTC processing of the sonic waveform data in the downhole environments.
Real-time delivery of the shear slowness has many applications, such as pore pressure and wellbore stability predictions. The information related to pore pressure and wellbore stability is very important for the driller to safely drill the well. One approach to accomplishing real-time sonic data processing is to compress the sonic waveform data so that they can be transmitted via telemetry. The compressed data are then used at the surface to reconstruct the sonic waveform data (decompression) for processing (e.g., STC processing). One example of such an approach is disclosed in U.S. Pat. No. 5,594,706 issued to Shenoy et al. Another example is found in a co-pending U.S. patent application Ser. No. 10/711,524 by Wu et al., filed on Sep. 23, 2004.
While these data compression methods may allow real-time processing of sonic waveform data, data compression has limitations. For example, some loss of information may not be avoidable if the data are highly compressed. Therefore, there is still a need for alternative methods that permit real-time processing of sonic waveform data.